β‑Diketiminato Nickel Imides in Catalytic Nitrene Transfer to Isocyanides

The β-diketiminato nickel­(I) species [Me₃NN]­Ni­(2-picoline) (<b>1</b>) serves as an efficient catalyst for carbodiimide (RNCNR′) formation in the reactions of a range of organoazides N₃R with isocyanides R′NC. [Me₃NN]­Ni­(CNR)₂ (R = ^tBu, Ar (Ar = 2,6-Me₂C₆H₃)) species provide carbodiimides RNCNAr′ upon reaction with Ar′N₃ (Ar′ = 3,5-Me₂C₆H₃). Nitrene transfer takes place via the intermediacy of nickel imides. Reaction of [Me_<i>x</i>NN]­Ni­(2-picoline) (<i>x</i> = 2 or 3) with Ar′N₃ gives the new dinickel imides {[Me_<i>x</i>NN]­Ni}₂(μ-NAr′) (<b>4</b> (<i>x</i> = 3) and <b>5</b> (<i>x</i> = 2)) as deep purple, diamagnetic substances. The X-ray structure of {[Me₂NN]­Ni}₂(μ-NAr′) (<b>5</b>) features short Ni–N_imide distances of 1.747(2) and 1.755(2) Å along with a short Ni–Ni distance of 2.7210(3) Å. These dinickel imides <b>4</b> and <b>5</b> react stoichiometrically with ^tBuNC to provide the corresponding carbodiimides ^tBuNCNAr′ in good yield. Azide transfer takes place upon reaction of <b>1</b> with TMS-N₃ to give the square planar nickel­(II) azide [Me₃NN]­Ni­(N₃)­(2-picoline) (<b>7</b>). Stoichiometric reaction of dinickel dicarbonyl {[Me₃NN]­Ni}₂(μ-CO)₂ with organoazides such as Ar′N₃ is sluggish, indicating that <b>1</b> is not an efficient catalyst for nitrene transfer from organoazides to CO to form isocyanates RNCO

Electrocatalytic Hydrogen Production by [Ni(7P^Ph₂N^H)₂]²⁺: Removing the Distinction Between Endo- and Exo-Protonation Sites

A new Ni­(II) complex, [Ni­(7P^Ph₂N^H)₂H]³⁺ (7P^Ph₂N^H = 3,6-diphenyl-1-aza-3,6-diphosphacycloheptane), has been synthesized, and its electrochemical properties have been reported. The 7P^Ph₂N^H ligand features an NH, ensuring properly positioned protonated amine groups (N–H⁺) for electrocatalysis, regardless of whether protonation occurs exo or endo to the metal center. The compound is an electrocatalyst for H₂ production in the presence of organic acids (p<i>K</i>_a range 10–13 in CH₃CN), with turnover frequencies ranging from 160 to 780 s^–1 at overpotentials between 320 and 470 mV, as measured at the potential of the catalytic wave. In stark contrast to [Ni­(P^Ph₂N^{R^′}₂)₂]²⁺ (P^Ph₂N^{R^′}₂ = 3,7-diphenyl-1,5-diaza-3,7-diphosphacyclooctane) and other [Ni­(7P^Ph₂N^{R^′})₂]²⁺ complexes, catalytic turnover frequencies for H₂ production by [Ni­(7P^Ph₂N^H)₂]²⁺ do not show catalytic rate enhancement upon the addition of H₂O. This finding supports the assertion that [Ni­(7P^Ph₂N^H)₂]²⁺ eliminates the distinction between the endo- and exo-protonation isomers

Electrocatalytic Hydrogen Production by [Ni(7P^Ph₂N^H)₂]²⁺: Removing the Distinction Between Endo- and Exo-Protonation Sites

A new Ni­(II) complex, [Ni­(7P^Ph₂N^H)₂H]³⁺ (7P^Ph₂N^H = 3,6-diphenyl-1-aza-3,6-diphosphacycloheptane), has been synthesized, and its electrochemical properties have been reported. The 7P^Ph₂N^H ligand features an NH, ensuring properly positioned protonated amine groups (N–H⁺) for electrocatalysis, regardless of whether protonation occurs exo or endo to the metal center. The compound is an electrocatalyst for H₂ production in the presence of organic acids (p<i>K</i>_a range 10–13 in CH₃CN), with turnover frequencies ranging from 160 to 780 s^–1 at overpotentials between 320 and 470 mV, as measured at the potential of the catalytic wave. In stark contrast to [Ni­(P^Ph₂N^{R^′}₂)₂]²⁺ (P^Ph₂N^{R^′}₂ = 3,7-diphenyl-1,5-diaza-3,7-diphosphacyclooctane) and other [Ni­(7P^Ph₂N^{R^′})₂]²⁺ complexes, catalytic turnover frequencies for H₂ production by [Ni­(7P^Ph₂N^H)₂]²⁺ do not show catalytic rate enhancement upon the addition of H₂O. This finding supports the assertion that [Ni­(7P^Ph₂N^H)₂]²⁺ eliminates the distinction between the endo- and exo-protonation isomers

C–H Functionalization Reactivity of a Nickel–Imide

We report bifunctional reactivity of the β-diketiminato Ni­(III)–imide [Me₃NN]­NiNAd (<b>1</b>), which undergoes H-atom abstraction (HAA) reactions with benzylic substrates R–H (indane, ethylbenzene, toluene). Nickel–imide <b>1</b> competes with the nickel–amide HAA product [Me₃NN]­Ni–NHAd (<b>2</b>) for the resulting hydrocarbyl radical R^• to give the nickel–amide [Me₃NN]­Ni–N­(CHMePh)­Ad (<b>3</b>) (R–H = ethylbenzene) or aminoalkyl tautomer [Me₃NN]­Ni­(η²-CH­(Ph)­NHAd) (<b>4</b>) (R–H = toluene). A significant amount of functionalized amine R–NHAd is observed in the reaction of <b>1</b> with indane along with the dinickel imide {[Me₃NN]­Ni}₂(μ-NAd) (<b>5</b>). Kinetic and DFT analyses point to rate-limiting HAA from R–H by <b>1</b> to give R^•, which may add to either imide <b>1</b> or amide <b>2</b>, each featuring significant N-based radical character. Thus, these studies illustrate a fundamental competition possible in C–H amination systems that proceed via a HAA/radical rebound mechanism

C–H Functionalization Reactivity of a Nickel–Imide

We report bifunctional reactivity of the β-diketiminato Ni­(III)–imide [Me₃NN]­NiNAd (<b>1</b>), which undergoes H-atom abstraction (HAA) reactions with benzylic substrates R–H (indane, ethylbenzene, toluene). Nickel–imide <b>1</b> competes with the nickel–amide HAA product [Me₃NN]­Ni–NHAd (<b>2</b>) for the resulting hydrocarbyl radical R^• to give the nickel–amide [Me₃NN]­Ni–N­(CHMePh)­Ad (<b>3</b>) (R–H = ethylbenzene) or aminoalkyl tautomer [Me₃NN]­Ni­(η²-CH­(Ph)­NHAd) (<b>4</b>) (R–H = toluene). A significant amount of functionalized amine R–NHAd is observed in the reaction of <b>1</b> with indane along with the dinickel imide {[Me₃NN]­Ni}₂(μ-NAd) (<b>5</b>). Kinetic and DFT analyses point to rate-limiting HAA from R–H by <b>1</b> to give R^•, which may add to either imide <b>1</b> or amide <b>2</b>, each featuring significant N-based radical character. Thus, these studies illustrate a fundamental competition possible in C–H amination systems that proceed via a HAA/radical rebound mechanism

C–H Functionalization Reactivity of a Nickel–Imide

We report bifunctional reactivity of the β-diketiminato Ni­(III)–imide [Me₃NN]­NiNAd (<b>1</b>), which undergoes H-atom abstraction (HAA) reactions with benzylic substrates R–H (indane, ethylbenzene, toluene). Nickel–imide <b>1</b> competes with the nickel–amide HAA product [Me₃NN]­Ni–NHAd (<b>2</b>) for the resulting hydrocarbyl radical R^• to give the nickel–amide [Me₃NN]­Ni–N­(CHMePh)­Ad (<b>3</b>) (R–H = ethylbenzene) or aminoalkyl tautomer [Me₃NN]­Ni­(η²-CH­(Ph)­NHAd) (<b>4</b>) (R–H = toluene). A significant amount of functionalized amine R–NHAd is observed in the reaction of <b>1</b> with indane along with the dinickel imide {[Me₃NN]­Ni}₂(μ-NAd) (<b>5</b>). Kinetic and DFT analyses point to rate-limiting HAA from R–H by <b>1</b> to give R^•, which may add to either imide <b>1</b> or amide <b>2</b>, each featuring significant N-based radical character. Thus, these studies illustrate a fundamental competition possible in C–H amination systems that proceed via a HAA/radical rebound mechanism

Hydrogen Production Using Nickel Electrocatalysts with Pendant Amines: Ligand Effects on Rates and Overpotentials

A Ni-based electrocatalyst for H₂ production, [Ni­(8P^Ph₂N^C₆H₄Br)₂]­(BF₄)₂, featuring eight-membered cyclic diphosphine ligands incorporating a single amine base, 1-<i>para</i>-bromophenyl-3,7-triphenyl-1-aza-3,7-diphosphacycloheptane (8P^Ph₂N^C₆H₄Br) has been synthesized and characterized. X-ray diffraction studies reveal that the cation of [Ni­(8P^Ph₂N^C₆H₄Br)₂(CH₃CN)]­(BF₄)₂ has a distorted trigonal bipyramidal geometry. In CH₃CN, [Ni­(8P^Ph₂N^C₆H₄Br)₂]²⁺ is an electrocatalyst for reduction of protons, and it has a maximum turnover frequency for H₂ production of 800 s^–1 with a 700 mV overpotential (at <i>E</i>_cat/2) when using [(DMF)­H]­OTf as the acid. Addition of H₂O to acidic CH₃CN solutions of [Ni­(8P^Ph₂N^C₆H₄Br)₂]²⁺ results in an increase in the turnover frequency for H₂ production to a maximum of 3300 s^–1 with an overpotential of 760 mV at <i>E</i>_cat/2. Computational studies carried out on [Ni­(8P^Ph₂N^C₆H₄Br)₂]²⁺ indicate the observed catalytic rate is limited by formation of nonproductive protonated isomers, diverting active catalyst from the catalytic cycle. The results of this research show that proton delivery from the exogenous acid to the correct position on the proton relay of the metal complex is essential for fast H₂ production

High Catalytic Rates for Hydrogen Production Using Nickel Electrocatalysts with Seven-Membered Cyclic Diphosphine Ligands Containing One Pendant Amine

A series of Ni-based electrocatalysts, [Ni­(7P^Ph₂N^C6H4X)₂]­(BF₄)₂, featuring seven-membered cyclic diphosphine ligands incorporating a single amine base, 1-<i>para</i>-X-phenyl-3,6-triphenyl-1-aza-3,6-diphosphacycloheptane (7P^Ph₂N^C6H4X, where X = OMe, Me, Br, Cl, or CF₃), have been synthesized and characterized. X-ray diffraction studies have established that the [Ni­(7P^Ph₂N^C6H4X)₂]²⁺ complexes have a square planar geometry, with bonds to four phosphorus atoms of the two bidentate diphosphine ligands. Each of the complexes is an efficient electrocatalyst for hydrogen production at the potential of the Ni­(II/I) couple, with turnover frequencies ranging from 2400 to 27 000 s^–1 with [(DMF)­H]⁺ in acetonitrile. Addition of water (up to 1.0 M) accelerates the catalysis, giving turnover frequencies ranging from 4100 to 96 000 s^–1. Computational studies carried out on the [Ni­(7P^Ph₂N^C6H4X)₂]²⁺ family indicate the catalytic rates reach a maximum when the electron-donating character of X results in the p<i>K</i>_a of the Ni­(I) protonated pendant amine matching that of the acid used for proton delivery. Additionally, the fast catalytic rates for hydrogen production by the [Ni­(7P^Ph₂N^C6H4X)₂]²⁺ family relative to the analogous [Ni­(P^Ph₂N^C6H4X₂)₂]²⁺ family are attributed to preferred formation of endo protonated isomers with respect to the metal center in the former, which is essential to attain suitable proximity to the reduced metal center to generate H₂. The results of this work highlight the importance of precise p<i>K</i>_a matching with the acid for proton delivery to obtain optimal rates of catalysis

High Catalytic Rates for Hydrogen Production Using Nickel Electrocatalysts with Seven-Membered Cyclic Diphosphine Ligands Containing One Pendant Amine

A series of Ni-based electrocatalysts, [Ni­(7P^Ph₂N^C6H4X)₂]­(BF₄)₂, featuring seven-membered cyclic diphosphine ligands incorporating a single amine base, 1-<i>para</i>-X-phenyl-3,6-triphenyl-1-aza-3,6-diphosphacycloheptane (7P^Ph₂N^C6H4X, where X = OMe, Me, Br, Cl, or CF₃), have been synthesized and characterized. X-ray diffraction studies have established that the [Ni­(7P^Ph₂N^C6H4X)₂]²⁺ complexes have a square planar geometry, with bonds to four phosphorus atoms of the two bidentate diphosphine ligands. Each of the complexes is an efficient electrocatalyst for hydrogen production at the potential of the Ni­(II/I) couple, with turnover frequencies ranging from 2400 to 27 000 s^–1 with [(DMF)­H]⁺ in acetonitrile. Addition of water (up to 1.0 M) accelerates the catalysis, giving turnover frequencies ranging from 4100 to 96 000 s^–1. Computational studies carried out on the [Ni­(7P^Ph₂N^C6H4X)₂]²⁺ family indicate the catalytic rates reach a maximum when the electron-donating character of X results in the p<i>K</i>_a of the Ni­(I) protonated pendant amine matching that of the acid used for proton delivery. Additionally, the fast catalytic rates for hydrogen production by the [Ni­(7P^Ph₂N^C6H4X)₂]²⁺ family relative to the analogous [Ni­(P^Ph₂N^C6H4X₂)₂]²⁺ family are attributed to preferred formation of endo protonated isomers with respect to the metal center in the former, which is essential to attain suitable proximity to the reduced metal center to generate H₂. The results of this work highlight the importance of precise p<i>K</i>_a matching with the acid for proton delivery to obtain optimal rates of catalysis

No effects of fingolimod on spinal cord glutamate transporter protein levels during EAE.

<p>(A-D) Western Blot analyses of spinal cord homogenates relative to GAPDH (A) or beta actin (C) with the respective densitometry relative to the housekeeping gene (B,D). At the maximum of EAE, there was a decrease for SLC1A2 (A,B) and SLC1A3 protein levels (C,D) as compared to naïve mice which was restored after fingolimod treatment (3 mg/kg once daily). One out of three experiments is shown, n = 3 per group. (E-H) Laser scanning microscopy of spinal cord cross sections after staining for GFAP (red) and SLC1A2 (green, E,F) or SLC1A3 (green, G,H). Arrows indicate double labelled profiles. Representative images of spinal cord cross sections are shown. Bar = 50 μm.</p